140 MINERALOGY AND LITHOLOGY. 



another again, but in the cold many stable compounds are formed in the 

 presence of one another which are impossible in the heat, while at the 

 same time entirely different mechanical agencies are brought to bear 

 upon matter. The original crust is now so deeply buried as to be inac- 

 cessible to the student; but it is plain that the nearest spot to which it 

 can be approached is the starting-point of lithology, and the study of the 

 various changes and modifications which this matter has passed through 

 will indicate the reason for the physical and chemical diversities which 

 are now so prominent. 



What rocks are most nearly like the earth's original crust is not hard 

 to decide. Chemistry points to a basic, siliceous rock, from theoretical 

 considerations ; and geologists find such rocks cutting through the oldest 

 formations, indicating that they came from a lower level. These rocks, 

 of which diabase and basalt are typical, are found with tolerably uniform 

 composition all over the world. Whether the rocks mentioned, or any 

 allied to them, are really composed of matter erupted from an unsolid- 

 ified zone of the earth's original substance, matters not here. These 

 rocks fulfil the conditions that must have existed, and at least represent 

 most nearly the first rocks from which all others have been derived. 

 With the study of the basic, eruptive rocks, our lithology therefore be- 

 gins. 



But it has been deduced, as a result of the labors of this survey, that 

 all our rocks are very old. A little area of Helderberg limestones repre- 

 sents the youngest of our stratified rocks, while the larger area is cov- 

 ered by archsean deposits, — and so all our rocks have been subjected to 

 the influence of ages of time. At the risk of being wearisome, in the 

 mineralogical chapter it was necessary to describe how almost every 

 mineral, when microscopically examined, was found to be subject to some 

 mode of decomposition pecuhar to itself; — therefore sections cut from 

 the various rocks of our state are continually presenting to us the pro- 

 cesses of decay and change by which old rocks are changed into new 

 ones; and, by illustrating as clearly as possible these processes, it is 

 hoped to simplify our lithology. 



It must be borne in mind that rock species, which are made of mixt- 

 ures of minerals which vary in their relative amounts, can have no such 

 definite boundaries as do minerals which are definite chemical compounds; 



