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through a circular opening which has been cut in their backs. Drive one or two of these flies into another vial, and introduce to them a wheat head having some fresh lice. See how the fly runs about them, examining them with its antennæ.
Having found one adapted to its wants, watch how dexterously it curves its body forward under its breast, bringing the tip before its face, as if to take accurate aim with its sting. There, the aphis gives a shrug, the fly has pricked it with its sting, an egg has been lodged under its skin, from which will grow a maggot like that first seen inside the dead swollen aphis. And thus the little fly runs busily around among the lice on the wheat heads, stinging one after another, till it exhausts its stock of eggs, a hundred probably, or more, thus ensuring the death of that number of these lice. And of its progeny, fifty it may be supposed, will be females, by which five thousand more will be destroyed. We thus see what efficient agents these parasites are in subduing the insects on which they prey. I find three different species of them now at work in our fields, destroying this grain aphis.”
Now whether these insects will appear in such numbers in coming years, or again be rarely seen ; and what are the causes of their great increase, we cannot tell. The army worm seems to be only an occasional visitor, and may not occur abundantly for several years. The aphis may be very troublesome for some years to come, and then gradually take its place among those insects which do each year considerable damage to grain. How to account for the sudden and marked increase of these or any other insects, once in five, twenty or fifty years, we do not know. Favorable or unfavorable seasons, the scarcity or abundance of parasites, and other physical influences that cause species to be rare or common in different seasons, do not satisfactorily to us, account for the fearful increase of the army' worm and grain aphis in the summer of 1861.
BY C. H. HITCHCOCK.
Inasmuch as this report is to be circulated chiefly among those who have never found time to devote much attention to scientific study, it has been thought by some that the elements of geology should constitute its chief portion. But little need be written preliminary to the account of our explorations the past season, because a general knowledge of geology is already possessed by intelligent men; and especially because four reports upon the Geology of Maine, made twenty years ago by Dr. C. T. Jackson, have already been distributed throughout the State. We will notice therefore only such fundamental features of the science as have an intimate connection with the details of the report.
Stratified and Unstratified Rocks. Geologists consider it settled that the earth was formerly a mass of molten matter similar to lava, assuming its present shape of a flattened sphere while its materials were thus plastic. As the medium in which the earth revolves is intensely cold, (at least 75 degrees below zero,) the tendency of the heated mass must have been constantly to throw off heat, and this at length to form a crust upon the surface of the spheroid. As soon as the crust became sufficiently thick for water to accumulate upon it, the process of denudation commenced, wearing away the higher parts of the envelop and depositing the detritus in the lower portions of the surface, or the valleys. At present the radiation of heat from the interior melted mass cannot be great, as the crust must be more than a hundred miles thick.
The original mass of the earth and all igneous rocks are said to
be unstratified, because they are not arranged in layers. On the other hand the rocks which have been formed by the deposition of the worn fragments of the original surface are said to be stratified, because they are arranged in layers, like a pile of boards. They have all been formed under water; either fresh water or marine. Many of the layers or strata are subdivided into very thin plates like the leaves of a book, which are called lamino. Slaty rocks are often traversed by divisional planes resembling laminæ, called cleavage planes, because the slates naturally cleave in the direction of these planes. Frequently the cleavage planes coincide with the laminæ. Another class of divisional planes called joints cross the strata in different directions and divide them into rhomboidal masses, parallelograms and cubes. The unstratified rocks are traversed by joints. Cleavage planes and joints are called superinduced structures because they have been developed by chemical forces since the original production of the rock.
Clay, sand, gravel and calcareous deposits, (such as are produced from mineral springs,) are the original forms of most of the stratified rocks. By various agencies these materials become consolidated into slates, sandstones, conglomerates and limestones ; so that the newer rocks are generally more friable and less compact than the older rocks. The slates, sandstones, etc., which have been formed under similar circumstances during one geological period, and contain relics of the animals and plants living during that period, are called a formation.
The strata composing the different formations were originally horizontal. Now an examination of the strata along a given line, or a geological section, shows that the stratified rocks are tilted up at various angles, and that some layers have been forced up so as to stand upon their edges. Some great force in nature seems to have been at work gradually for ages, pushing the strata into great curves, of which the strata we examine in a section are parts. Hence it is easy to see how one is enabled to examine the oldest stratified rocks and all that have been formed since the earliest period. One formation has been elevated so that the character of its layers can be ascertained at the surface, and the formations which have been deposited since lie upon the older formation without conccaling it entirely from view. Had there been no disturbances of the earth's crust, and had the formations encircled the earth one over another, like the concentric coats of an onion, it would
have been impossible to examine the oldest formation without excavating a passage through all the layers above it. Occasionally one formation has been elevated or tilted up before the formation immediately succeeding has been deposited. In such case the strata beneath are said to underlie unconformably the strata above with a less inclination.
Fossiliferous and Unfossiliferous Rocks. An easy and natural division of the stratified rocks is into those which contain evidences of organic life and those without fossils—or the fossiliferous and anfossiliferous or Azoic rocks. A fossil may be defined as any relic or trace of an animal or plant buried naturally in the earth. In general, the unfossiliferous rocks are the oldest, of immense thickness, but not so thick as the ten or twelve miles of fossiliferous rocks overlying them.
Paleontology is the science of fossils or organic remains. From the study of fossils the following general statements have been deduced.
1. The whole number of species of animals and plants found fossil and described amounts to at least 35,000, of which 2,750 are plants.
2. Each formation is characterized by its peculiar group of fossils not found in any other, so that a paleontologist on seeing a specimen can usually tell from what part of the series it came.
3. The fossil species in Maine and the northern parts of the continent correspond more nearly to animals now living in the tropics, than to those now living in the colder climates. Hence the climate of even the Arctic zone must once have been tropical.
4. There have been at least six entirely different races of animals and plants upon the globe; and some authors estimate as many as twenty-seven life periods. Each economy of life seems to have entirely disappeared before the appearance of the succeeding race, and each successive group must have been introduced, as was the first one, by the action of an intelligent creator. 5. There are no animals now existing at all similar to many
of the strange forms peopling the Preadamic world.
6. The successive groups of animals and plants are each more perfect than the one preceding : and the last group, or those now living, embraces the highest types of the animal and vegetable kingdoms.
Classification of Rocks. The fossiliferous rocks may be divided into three great systems, according to the times when the successive economies flourished:
1. The Paleozoic system of life, or the ancient—the oldest_types of being : 2. The Mesozoic system of life, or the types that flourished during the middle periods of organic life : and 3. The Cainozoic system of life, or the recent types of life. These are only general divisions. The subdivision of strata has now been carried to great nicety.
A formation in America is identified with the corresponding strata in Europe by means of the organic remains characteristic of that group. Previously the European strata had been examined, and had received a local name, giving it a definite place upon the series. For example, one of the lowest formations of the Paleozoic system was first explored in Wales. Hence it was called Silurian, (from an ancient name of the country.) Now when strata of the same age are found in America they are termed Silurian, i. e. strata of the same age with those in Siluria. The term Silurian is yet a general one. It has been subdivided into thirteen parts in England, and into sixteen in North America, each being named from the town or region where it is best developed.
The unfossiliferous stratified rocks, as well as the unstratified, are at present classified according to their lithological structure.
We present here a tabular view of all the stratified and unstratified rocks, arranging the former in the order of their age, mentioning the most recent first.
A.-STRATIFIED OR AQUEOUS ROCKS.
3. Paleozoic. 1. Permian.
4. Upper Silurian. 2. Carboniferous.
5. Lower Silurian. 3. Devonian, or Old Red Sandstone. 6. Cambrian, (or Huronian.)
II.-UNFOSSILIFEROUS OR Azoic.