3!)"2 II. S. WASHINGTON — IGNEOUS COMPLEX OF MAGNET COVE 



basic rocks prevent any very satisfactory examination. Railroad cuts 

 and quarries (except the Diamond Jo) are not to be found, and the 

 streams have not eroded in such a way as to help us materially. 



The difficulty of exact determination thus arising is frequently re- 

 marked on by Williams,* who, on this account, does not claim great 

 accuracy in the details of his map, especially in the dikes, the positions 

 of many of which could only be inferred by boulder trains, though the 

 main areas are quite certain. 



Williams' vieivs. — The explanation which Williams offers of the re- 

 lationships of the various rocks may be given in his own words,t a few 

 unimportant phrases being omitted : 



"The igneous rocks of Magnet Cove are divided into three genetically distinct 

 groups, whose structure and mode of occurrence show that they were formed dur- 

 ing three distinct periods of igneous activity. The oldest of these consist of the 

 basic, eleolitic, abyssal rocks which constitute a large part of the interior Cove 

 basin. The large masses of these rocks are holocrystalline granitic in their struct- 

 ure and were cooled slowly and under pressure. During the next period the rock 

 in and about the Cove, which had heen disturbed and heated by the intrusion of 

 the masses of abyssal rocks, cooled and cracks opened in all directions. These 

 cracks are filled with monchiquitic rocks. The third and last period of activity is 

 that in which the eleolitic and leucitic rocks of the " Cove ring " were formed, and 

 during which the numerous tinguaitic dikes of all varieties were intruded. The 

 rocks of this period are all of an intrusive character, a fact which is shown by their 

 structure and mode of occurrence. These youngest rocks cut both the abyssal 

 rocks and the dikes of monchiquite, and are therefore younger than either of these 

 groups." 



The facts observed in the field and in the laboratory do not seem to 

 me to bear out this idea of three genetically distinct groups of rocks 

 and three distinct periods of intrusion. On the contrary, they point 

 rather to the view that the rocks are all genetically connected, and that 

 the mass as a whole is probably a laccolith, and was, at any rate, formed 

 by a single intrusion of magma, which differentiation has split up into 

 two main groups of central, basic, ijolitic rocks, and peripheral, less basic, 

 syenites, the monchiquitic and tinguaitic dikes being both contempora- 

 neous and subsequent injections of these differentiates into the cracked 

 cover and cooled igneous mass and surrounding rocks. 



Form of the area. — The igneous area has an approximately circular or 

 rather broadly elliptical shape, the two axes measuring about 5 and 3 

 kilometers, the direction of the major axis being about west-northwest 

 by east-southeast. The central portion or " Cove basin " is low — from 



♦Williams : Op. eit., pp. 17:S-2(>1 passim, 

 f Williams: Op. eit., p. 342. 



