STRUCTURAL GEOLOGY. 43 



Oldest quartz-porphyry.] 



- 



the ocean to bring potassium within the bounds of possible condensation and precipita- 

 tion.* This was an inference from the observed later introduction of acid-alkaline 

 rocks in the Archean than the ferro-magnesian. It is a striking coincidence with 

 that argument that, here, the oldest known acid rock not only shows signs of oceanic 

 agency, but also embraces, along with phenocrysts of quartz, those of orthoclase, and 

 that the fine matrix of these phenocrysts is both siliceous and alkaline, in a state 

 of fine crystallization. 



The heated waters of the Archean ocean had, at the date of this precipitation, 

 been able to accumulate only a coarse "mud-conglomerate," a stratum seen at this 

 place to have a thickness of about 105 feet, made up wholly of roundish and more or 

 less squeezed greenish pebbles. This band sometimes has been designated a "green- 

 stone conglomerate " from the nature of its pebbles, but in other cases it has been 

 styled "mud conglomerate," from the fineness of their grain and the smoothness of 

 their outlines, which also indicates an original plasticity. Under such conditions 

 not only would the alkaline ocean hold in solution much silica, but such silica would, 

 with alumina (also present as a product of decay and dissolution of the greenstone 

 crust), necessarily unite to form such minerals as orthoclase. Hence would result, 

 posssibly, from a supersaturated solution, a rock as an oceanic product which is 

 usually considered a normal igneous rock resulting from fusion. 



Even with this origin for the earliest acid rock, which is here a quartz-porphyry, 

 this rock is not far removed from the same operations and the same agencies as those 

 which are called igneous, for heat and moisture and plasticity are the essential con- 

 ditions for the production of all acid igneous rocks, unless they be derived from some 

 primordial magma. This explanation differs only in requiring a lower degree of 

 heat (less than the boiling point of water), a longer period for crystallization and an 

 enormous scale of operation, as contrasted with the restricted limits of normal 

 volcanic action. It also implies the formation of the rock over broad expanses of 

 the ocean's bed rather than in the reservoirs of the earth's crust; and as a corollary 

 it points to the porphyritic rather than the granitic as the structure assumed by the 

 earliest acid rocks. 



Whence the Archean ocean obtained this large supply of potassium, it is not 

 necessary to inquire. Whether it was imprisoned at greater depths within the earth, 

 and for that reason did not appear in the ferro-magnesian earliest crust, but was 

 thrown out by later volcanic action, and thus reached the sea, or was retained still 

 in the atmosphere and was precipitated first into the ocean and later was thrown 

 down as alkaline silicates as above indicated, is subject for legitimate further specula- 

 tion, but it is not within the scope of this discussion. It might be added, however, 



*The origin of the Archean igneous ro;:ks. .1 >ii'-rirn n (fftloijist, xxii, 299, 1898. 



