THE ROCK RELATIONS OF THE WALKING FERN 



By E. J. Hill. 



IN 1SS9 I collected Camptosorus rhizophyllus at St. Croix 

 Falls, Wis. It was growing on a detached mass of trap that 

 had fallen from a ledge by the border of a lakelet just below 

 the village and was probably the same station from which it was 

 obtained by Dr. C. C. Parry in 134S when a member of Owen's 

 Geological Survey of Iowa, Wisconsin and Minnesota. He gives 

 as the habitat, " shaded and detached rocks, Falls of St. Croix." 

 The kind of rock is not mentioned as in the case of some ferns 

 found here or in other places, one of them Woodsia obtusa, " on 

 trap rocks at St. Croix Falls," which was frequent in the same 

 situations in 1889. As trap is a comprehensive term in lithology 

 it is well to specify that the rocks of this kind at the Dalles of the 

 St. Croix River are mostly melaphyre. or diabase porphyry. 



On inspecting the list of rocks on which the Walking Fern 

 grows, as given in the note in Fern Bulletin for October, the 

 question- naturally arises whether there is any mineral substance 

 common to all of them which this fern might select. A chemical 

 analysis would be the best test if the fern is led to absorb a par- 

 ticular kind. The rocks mentioned are gneiss, granite, quartzite, 

 sandstone, shale and limestone, and to them is now added trap 

 (melaphyre). It is so wide a range that the preference of the fern 

 would seem to be for rock rather than for any particular in- 

 gredient. 



Williamson in " Ferns of Kentucky " says: " It is of easy culti- 

 vation, either in mounds or in the Wardian case," but without 

 specifying whether on rock or ordinary soil. The rocks that have 

 been mentioned are silicious with the exception of shale and lime" 

 stone. Shale, like trap, is a generic term, and though commonly 

 aluminous is often arenacious, and the clays that come from their 

 decomposition are rarely so pure as to be free from sand particles. 

 Where I have found the walking fern on limestone, as at LeRoy, 

 N. Y., the rocks were cherty, abounding in nodules of rlint. This 

 is true also of the limestone in the vicinity of Joliet, 111., where 

 the fern occurs, they being frequently cherty and often magnesian, 

 the silicified remains of corals and other animals of the ancient 

 seas being frequent in them. Limestone often contains clay, sand 

 and other mineral impurities, either in pockets or mingled in their 

 mass, as the names argillaceous, and arenaceous limestones indi- 

 cate. 



