GEOLOGY OF SAX FRANCISCO PEXIXSULA 73 



much of which now recognizable, is of quite recent date, geo- 

 logically speaking. 



In the last sentence quoted Professor Lawson seems to recog- 

 nize quite fully the complicated structure of the Franciscan 

 series. 



The serpentine in the area covered by Professor Lawson's 

 report is divided into three linear tracts. One extends across 

 the city of San Francisco from Fort Point to Hunters Point, a 

 distance of ten miles, with a width of one and one-half miles. 

 Another large tract extends from San Andreas Lake to San 

 Mateo Creek, having a length of eleven and one-half miles and 

 a maximum width of one mile. 



According to the description these bodies occur as laccolites, 

 laccolitic sills, or dikes. From a careful perusal of the report, 

 and an examination of several of the more important areas 

 covered by these eruptives, I cannot understand the reason for 

 applying to them the designation laccolite. This term was 

 originall}' given by Gilbert to an eruptive mass which in the 

 course of being forced upward, instead of reaching the surface, 

 finally spread out between the strata, forming a thick lens, and 

 arching them over it in dome form. Professor Lawson is certainly 

 right in the statement that the main bodies are not true dikes, 

 but it seems to me that the term sheet or sill which he uses quite 

 frequently is the really proper one for these eruptives. If the 

 term laccolite has any exact meaning it is certainly not synony- 

 mous with sheet or sill. 



I have carefully examined the so-called laccolites of Hunters 

 Point, Potrero and Fort Point and can find no evidence that 

 these eruptive masses were ever covered by an arched roof. In 

 my opinion the field relations indicate that they cooled as sheets 

 or dikes. They have no appearance of being lens shaped, and 

 if it is true, as Professor Lawson supposes, that the}' are all con- 

 nected through the distance of ten miles, we have then a long sheet 

 inclined at an angle of 35° to 40°; and though in a general way 

 intruded along the bedding planes of the sandstone and shale, 

 yet, owing to the marked irregularities of the surface of the erup- 



