The Diahasic Schists — H, V. Winch ell. 
21 
basic rock it has been hardened and rendered massive and 
porphyritic. It gradually resumes its original character as we 
recede from the greenstone ridge. 
That this diabase is of an old date is proved by later eruptive 
greenstones which intersect it in the form of dykes. These are 
composed of fresher looking rock, and though they are occa¬ 
sionally faulted, have evidently not undergone such epochs of 
pressure and disturbance as the greenstone which they cut. 
Following the diabase again north-westwardly across the 
strike it becomes schistose and less like an eruptive rock. 
Grains of free silica may be seen here and there and signs of 
sedimentary arrangement are more numerous. These changes 
go on until the rock is found to be siliceous and even flinty, 
and has unmistakable sedimentary bands, or is a smooth, soft 
argillyte of a gray or nearly black color. 
The above very condensed account is intended to bring into 
notice two points regarding the origin and the mutual relations 
of the jaspilyte and diabasic schists. First , the jaspilyte is con¬ 
tained in a basic eruptive rock which is frequently conglom- 
eritic and generally schistose; but which is at times perfectly 
massive and crystalline. Second , this basic rock has in many 
places undeniable evidences of aqueous arrangement, and grades 
conformably into siliceous stratified rocks which are purely 
fragmental. 
To explain how a basic, eruptive rock can thus change by in¬ 
sensible gradations into a siliceous fragmental one is not a diffi¬ 
cult matter. 
Suppose a region, as in north-eastern Minnesota, which for a 
long period was the seat of basic volcanic eruptions. The 
Sandwich islands and the island of Java furnish examples of 
similar regions existing at the present time. In the shallow 
ocean there would be formed islands by the volcanic accumula¬ 
tions, and others by elevations of the earth’s crust. These ex¬ 
posed districts would be subject to erosion and would furnish 
acidic sediments and siliceous deposits. From the craters erup¬ 
tions would take place and sheets of lava would flow down the 
sides of the volcanoes into the sea or over the land as the case 
might be. At the same time volcanic dust and ashes would be 
thrown up to a great height and carried perhaps for miles be¬ 
fore they were deposited in the surrounding waters. Accom- 
