LITHOLOGY. I4I 



but, on the contrary, by the gradual introduction of new minerals, and 

 the elimination of others, they so gradually approach and grade into one 

 another as to make it a matter of personal judgment where the dividing 

 line must be drawn. Hence, the science of lithology is unsatisfactory 

 and puzzling to those who pursue it with the idea of the classification 

 and nomenclature of their specimens in the foreground, while the indefi- 

 nite limits of species, and the gradations that occur between different 

 rocks, are aids to those who study the subject with the idea of discov- 

 ering the nature and origin of rock types. 



The study of a rock begins in the field ; and though in the laboratory 

 the student with his microscope can surmise many facts that are properly 

 ascertained where the rocks are in place, the necessity of field work is 

 not at all diminished. The first point to be noted in the field is the rela- 

 tionship of the given rock to those about it, and on this relationship the 

 chief division of rocks is founded, Fragmental rocks are masses of loose 

 or merely cemented sedimentary materials. The crystalline schists are 

 masses of sediments, the materials of which have been rearranged in 

 crystalline form. These two kinds of rocks show plain evidences of 

 stratification. Intrusive rocks are those that bear no relationship to 

 those about them, save that they form dykes or veins in them, and can- 

 not be said to belong to the formations in which they occur. Between 

 these groups of rocks, the members of which are either plainly stratified 

 or plainly intruded, there is a group of rocks which are subject to dis- 

 cussion. At times they appear to be stratified, at times they are plainly 

 intrusive ; but more often they show plain evidence of neither one nor 

 the other. In New Hampshire all these groups are very fully repre- 

 sented. Intrusive rocks are constantly met with, stratified rocks are 

 everywhere, while there are members enough of the intermediate group, 

 the relationship of which it is the duty of a treatise of this kind to dis- 

 cuss. It will thus be seen, that, as the boundary lines between species 

 are indistinct, so, too, are those that divide the great classes from one 

 another. This will make still more plain the force of what has been said 

 on the subject of classification, while it may again be said that this indefi- 

 nite division is a help towards the understanding of rocks in general; 

 for it is easier to travel a smooth road than to spring from stone to 

 stone. As an adjunct to geology, the difficulties with which lithology 



